FIG. 1 shows a network structure of the E-UMTS, a mobile communication system, applicable to the related art and the present invention. The E-UMTS system has been evolved from the UMTS system, for which the 3GPP is proceeding with the preparation of the basic specifications. The E-UMTS system may be classified as the LTE (Long Term Evolution) system.
The E-UMTS network may be divided into an evolved-UMTS terrestrial radio access network (E-UTRAN) and a core network (CN). The E-UTRAN includes a terminal (referred to as ‘UE (User Equipment), hereinafter), a base station (referred to as an eNode B, hereinafter), a serving gateway (S-GW) located at a termination of a network and connected to an external network, and a mobility management entity (MME) superintending mobility of the UE. One or more cells may exist for a single eNode B.
FIG. 2 and FIG. 3 illustrate a radio interface protocol architecture based on a 3GPP radio access network specification between the UE and the base station. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane for transmitting data information and a control plane for transmitting control signals (signaling). The protocol layers can be divided into the first layer (L1), the second layer (L2), and the third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in communication systems.
The radio protocol control plane in FIG. 2 and each layer of the radio protocol user plane in FIG. 3 will now be described.
The physical layer, namely, the first layer (L1), provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to an upper layer called a medium access control (MAC) layer via a transport channel, and data is transferred between the MAC layer and the physical layer via the transport channel. Meanwhile, between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transferred via a physical channel.
The MAC layer of the second layer provides a service to a radio link control (RLC) layer, its upper layer, via a logical channel. An RLC layer of the second layer may support reliable data transmissions. A PDCP layer of the second layer performs a header compression function to reduce the size of a header of an IP packet including sizable unnecessary control information, to thereby effectively transmit an IP packet such as IPv4 or IPv6 in a radio interface with a relatively small bandwidth.
A radio resource control (RRC) layer located at the lowest portion of the third layer is defined only in the control plane and handles the controlling of logical channels, transport channels and physical channels in relation to configuration, reconfiguration and release of radio bearers (RBs). The radio bearer refers to a service provided by the second layer (L2) for data transmission between the UE and the UTRAN.
As mentioned above, the base station and the UE are two main entities constituting the E-UTRAN. Radio resources in a single cell include uplink radio resources and downlink resources. The base station handles allocating and controlling of uplink radio resources and downlink radio resources of the cell. Namely, the base station determines which UE uses which radio resources at which moment. For example, the base station may determine to allocate frequency from 100 Mhz to 101 Mhz to a user 1 for downlink data transmission in 3.2 seconds. After such determination, the base station informs the UE accordingly so that the UE can receive downlink data. Also, the base station may determine when and which UE is allowed to transmit uplink data by using which and how many radio resources, and then informs a corresponding UE accordingly, so that the UE can transmit data by using the radio resources for the corresponding time. In the related art, a single terminal keeps using a single radio resource during a call connection, which is irrational for the recent services which are mostly based on IP packets. That is, in most packet services, packets are not continually generated during a call connection but there are intervals in the call during which none is transmitted. Thus, continuously allocating radio resources to the single terminal is ineffective. To solve this problem, the E-UTRAN system employs a method in which radio resources are allocated to the UE in the above-described manner only when the UE requires it or only when there is service data.
In general, a dynamic radio resource scheduling is a method for informing radio resources to be used every time of a transmission or reception of UE. FIG. 4 is an exemplary view showing the operations of the dynamic radio resource allocation. Typically, an uplink radio resource allocation (e.g., UL GRANT) message or downlink radio resource allocation (e.g., DL ASSIGNMENT) message is transmitted via a Physical Downlink Control Channel (PDCCH). Accordingly, a UE receives or monitors the PDCCH at every designated time. Upon receiving a UE identifier (e.g., C-RNTI) allocated, then the UE receives or transmits radio resources indicated in the UL GRAT or DL ASSIGNMENT transmitted via the PDCCH, and then uses the radio resources to enable data transmission/reception between the UE and eNode B.
In more detail, in the LTE system, in order to effectively use radio resources, the base station should know which and how many data each user wants to transmit. In case of downlink data, the downlink data is transferred from an access gateway to the base station. Thus, the base station knows how many data should be transferred to each user through downlink. Meanwhile, in case of uplink data, if the UE does not directly provide the base station with information about data the UE wants to transmit to uplink, the base station cannot know how many uplink radio resources are required by each UE. Thus, in order for the base station to appropriately allocate uplink radio resources to the UEs, each UE should provide information required for the base station to schedule radio resources to the base station.
To this end, when the UE has data to be transmitted, it provides corresponding information to the base station, and the base station transfers a resource allocation message to the UE based on the received information.
In this process, namely, when the UE informs the base station that it has data to be transmitted, the UE informs the base station about the amount of data accumulated in its buffer. It is called a buffer status report (BSR).
The BSR is generated in the format of a MAC control element, included in a MAC PDU, and transmitted from the UE to the base station. Namely, uplink radio resources are required for the BSR transmission, which means that uplink radio resource allocation request information for BSR transmission should be sent. If there is allocated uplink radio resource when the BSR is generated, the UE would transmit the BSR by using the uplink radio resource. The procedure of sending the BSR by the UE to the base station is called a BSR procedure. The BSR procedure starts 1) when every buffer does not have data and data is newly arrived to a buffer, 2) when data is arrived to a certain empty buffer and a priority level of a logical channel related to the buffer is higher than a logical channel related to the buffer previously having data, and 3) when a cell is changed. In this respect, with the BSR procedure triggered, when uplink radio resources are allocated, if transmission of all the data of the buffer is possible via the radio resources but the radio resources are not sufficient to additionally include the BSR, the UE cancels the triggered BSR procedure.
Here, a power headroom report (PHR) may also exist apart from the BSR. The power headroom report notifies or indicates how much additional power can be used by the terminal. Namely, the PHR may represent a power offset between a most capable transmitting power of the terminal and a current transmitting power of the terminal. This can be also defined as the difference between a nominal UE maximum transmit power and an estimated power for UL-SCH transmission.
The main reason that the terminal transmits the PHR to the base station is to allocate a proper amount of radio resources for the terminal. For example, it is assume that a maximum transmit power of the terminal is a 10 W and the terminal currently uses a 9 W power output using a 10 Mhz frequency range. If a 20 Mhz frequency range is allocated to the terminal, the terminal needs an 18 W power (9 W×2). However, as the maximum transmit power of the terminal is limited to the 10 W, if the 20 Mhz frequency range is allocated to the terminal, the terminal can not use entire frequency range, or, due to the lack of the power, the base station can not receives a signal from the terminal.
Most of current communication traffics are on basis of an Internet service in modern technologies. And, one characteristic of data used in the Internet service is that these data are suddenly generated without any anticipation. Further, an amount of generated data is also bursty and unpredictable. Therefore, in case that the terminal suddenly has data that is need to be transmitted, if the base station has information related to the PHR from the terminal beforehand, it will be much easily for the base station to allocate a proper amount of radio resources for the terminal. Here, the PHR itself is not transmitted to the base station with a reliable manner. Namely, all PHR transmitted from the terminal, are not successfully received by the base station. Therefore, in related art, a periodic PHR transmission is used. Specifically, the terminal operates a timer (i.e. a periodic PHR timer), and transmits the PHR to the base station whenever the timer expires.
In the related art, the terminal triggers a periodic PHR when the periodic timer is expired. If the periodic PHR is actually transmitted, the terminal restarts the periodic timer. Here, the PHR is also triggered when a path loss measured by the terminal changes more than a threshold value.
As aforementioned, the terminal transmits a new PHR to the base station when a periodic timer expires, and then the terminal restarts the periodic timer periodically. Also, the terminal continuously monitors a path loss, and then the terminal transmits a new PHR when the monitored path loss changes more than a threshold value.
FIG. 5 is an exemplary view of transmitting a power headroom report (PHR) according to the related art. As depicted in the FIG. 5, if a new PHR transmitted time due to the path loss changes and a new PHR transmitted time due to the expiration of the periodic timer is relatively close, path loss thereafter is not significantly changed. Accordingly, information contained in the new PHR due to the expiration of the periodic timer is not much different from information contained in the new PHR due to the path loss changes. This may cause a great amount of radio resources waste. Namely, in the related art, there is a drawback of using unnecessary radio resource(s) during a PHR transmission procedure.